Speed of Sound in Materials: Impact on "Vaporization" Gun

[Jota]

"vaporization" gun

I got my information on speeds of sound through various materials from this page: http://hyperphysics.phy-astr.gsu.edu/hbase/tables/soundv.html#c1

Material v (m/s)
Diamond 12000
Pyrex glass 5640
Iron 5130
Aluminum 5100
Brass 4700
Copper 3560
Gold 3240
Lucite 2680
Lead 1322
Rubber 1600



This might sound a little crazy, but I'm a kind of 'curiousity-addict', so I'll ask anyway: If one of these substances were placed in the firing chamber of some kind of gun, a projectile placed in front of it, and then said substance was vaporized, would it expand at the same speed as sound travels through them, and therefore propel the projectile at that speed? Assuming of course both gun and projectile were made of an exotic material strong enough to withstand the friction.

 

[mgb_phys]

Not necessarily.
The speed of sound is the fastest an impulse can travel through a material, so if you were trying to vaporise it by hitting one end very hard then the shock wave would travel at the speed of sound through the unmelted part.

You could apply enough heat around the entire projectile to vapourise it quickly and then let the expanding gases propel the projectile.
This wouldn't depend on the speed of sound but would be limited by the poor conduction from the evaporating part to the still solid centre.
You should probably look for a material with low molecular mass (so you get as many moles of gas as possible) low melting point and low specific heat capcaity.
A similair system is already used to make very high speed air guns for ballistics research. A block of plastic is heated very quickly by powerful x-rays to generate gas which fires a projectile.
I'm not sure why this is used rather than explosives, it might be that the vaporisation of the plastic is faster than typical explosive velocities or it might be to keep a chemically pure atmosphere without the combustion products of an explosive.